A digital loop carrier (DLC) carries plural voice and/or data channels over a number of lines. A DLC comprises a Central Office Terminal (COT), a remote terminal (RT), and multiple subscriber lines. The RT contains channel units with line cards and serves to connect individual subscriber lines to the COT. The transmit direction of a DLC comprises voice frequency signals entering the channel units from the subscriber premises. These voice frequency signals produce pulse amplitude modulation (PAM) samples at an 8 kHz rate. This sampling of channels is performed in a fixed sequence, producing a stream of pulses. Each PAM pulse is encoded into a pulse code modulation (PCM) word by the channel unit coder/decoder (CODEC) therein.
A transmit/receive unit (TRU) at an RT, for example, polls the plurality of channel units therein for PCM words using sequential polling. The TRU time division multiplexes the PCM words into a bitstream and transmits the bitstream to a line interface unit (LIU). The LIU converts the bitstream from the TRU into a T1 signal. This T1 signal is then transmitted to the COT.
Regarding the reception of the signals from an RT by the subscriber premises, the LIU performs framing on the T1 input from a line and sends it to the TRU. The TRU then distributes the PCM to the channel units which decode the PCM into corresponding PAM pulses. The voice frequency signals are then reconstructed, and sent over a twisted pair line to the subscriber.
In a T1 digital carrier system, for example, a voice signal is sampled, resulting in a PAM signal that is converted to an 8-bit PCM digital signal, and then interleaved with 23 other channels for transmission over a T1 line at a bit stream rate of 1.544 megabits per second (Mbps). T1 signals are processed in a channel bank such as the channel bank 50 depicted in FIG. 1. In a conventional digital channel bank 50, twenty-four channels are collectively referred to as a digroup. Channel banks typically comprise two digroups A and B to create a 48-channel framework for transmitting and receiving on two duplex T1 carriers. As shown in FIG. 1, the channel bank 50 comprises a chassis 90 having physical card slots 60 into which at least forty-eight channel unit (CU) cards 80 can be inserted, as well as a number of common equipment cards 70.
A disadvantage of current telephone systems is evident when a subscriber receives an incoming call and wishes to switch telephones. The subscriber places the first telephone off-hook and switches to a second telephone connected to the same line, thus taking a second telephone off-hook. In such a situation, if the subscriber does not place the first telephone on-hook after terminating the call, the line will be considered in use and unavailable for subsequent incoming or outgoing calls. Therefore, a need exists to allow for switching telephones, during an incoming or outgoing call, by placing a first telephone on-hook and subsequently taking a second telephone off-hook, yet not losing the initial incoming or outgoing call.
Some telephones have the ability to allow a subscriber to enable/disable an on-hold function while the subscriber answers another telephone line or switches to another telephone that is also provided with an on-hold enable/disable function. This on-hold feature is accomplished via components located within the telephone, and is not available to other telephones at the subscriber premises if they lack the internal components to support this function. Therefore, a need exists to allow remote processing (e.g., at a remote terminal or a central office) of a series of dual-tone multi-frequency (DTMF) codes to prevent termination of a call while a first telephone is placed on-hook and, subsequently, a second telephone is placed off-hook. Thus, existing special internal components for the on-hold function, which are not typically available in a telephone, are not needed to take advantage of this feature.
Since one RT serves a plurality of subscriber premises, it would be convenient for telephone companies to offer the above mentioned off-hook management to its customers by means of the channel unit. This would require no additional special equipment in subscribers' telephones or homes.